Today's industry has been developed almost completely based on fossil fuels. However, the environmental pollution resulting from the development of the industry has recently become a serious social problem.
The environmental pollution mentioned above can be classified broadly into air pollution, water pollution and soil contamination. In particular, the problem of soil contamination is a very serious threat to food production, inducing water pollution through contamination of groundwater. It is not easy to solve the problem of soil contamination as compared with the air pollution and the water pollution.
Among fossil fuels, oil such as petroleum or the like can be used in various industrial fields and is utilized extensively. However, for a variety of reasons such as a marine oil spill accident, carelessness in transportation and storage, and the like, a considerable amount of oil may be drained into soil, thereby causing serious soil contamination.
There is a growing interest in the techniques for artificially purifying soil contaminated with oil, because a considerable period of time is required for natural recovery of soil contaminated with oil.
Techniques such as soil washing, thermal desorption, bio-remediation and the like have been proposed as conventional methods for purifying soil contaminated with oil.
Specifically, the soil washing technique is a technique of separating contaminants from the surface of soil using water or surfactant.
In the soil washing technique, when treating soil contaminated with heavy oil or crude oil, there is problem in that the consumption of surfactant is uneconomically excessive. If water makes contact with heavy oil or crude oil, there is a problem that shows a phenomenon in which tar balls are generated and a screen is clogged by the tar balls or a phenomenon in which the contact area of a surfactant decreases and the treatment effect decreases abruptly.
The thermal desorption technique is a technique that purifies soil by a process of applying heat to soil contaminated with oil, volatilizing oil contaminants and then burning the volatilized contaminants in a combustion chamber (burner). When the volatilized contaminants are burnt in the combustion chamber, dust is generated. The dust thus generated is discharged into the atmosphere after contaminants are minimized by filtering the dust with a bag filter.
In the thermal desorption technique, when burning the contaminants volatilized from contaminated soil, oxygen is required in proportion to the carbon number of contaminants. In the case where the causative substance contaminating the soil is light oil having a relatively small carbon number, there is no problem in purifying soil by the thermal desorption technique. However, if soil is contaminated with heavy oil having a relatively large carbon number, the amount of oxygen required for burning contaminants becomes very large. The bag filters need to be increased in proportion thereto. This poses a problem in that a post-treatment facility becomes very excessive.
The bio-remediation technique is a purification technique which makes use of microorganisms. According to the EPA of US, it is generally possible to treat TPH up to 5,000 mg/kg. Typically, TPH of soil contaminated with heavy oil or crude oil exceeds 5,000 mg/kg. Thus, the treatment with microorganisms has a limitation. As used herein, the term “TPH” is an abbreviation of Total Petroleum Hydrocarbon. The higher the numerical value of TPH, the greater the degree of soil contamination.
If the soil contaminated with crude oil is left for decades, light oil components with a relatively small carbon number in the crude oil are removed in their entirety and only the hardly-degradable heavy oil components are left. It may be difficult to biologically decompose the heavy oil components. In a dry environment where the sunlight is strong, it is practically difficult to secure the supply of moisture and the breathability required for microorganisms. There is a limitation in that it is difficult to apply a bio-remediation process.